Project Summary Dental materials are among the most ubiquitous medical devices, with almost all adults in the modern world benefiting from their use. Yet, despite their commonality the most employed composite, a methacrylate based polymer resin, faces many issues such as high shrinkage and shrinkage stress, water sorption, hydrolytic and enzymatic degradation, and is prone to leaching of unreacted monomer. While the composite does cure rapidly as is ideal for clinical settings, meticulous procedures and environments are required. The composite must be cured in increments of 2-3 mm at a time to assure adequate cure and prevent stress development. Further, adhesion of the composite depends on keeping the area completely dry and even if all these conditions are met, the composite is not a permanent solution with secondary caries often recurring. Thus, while composite materials have been used in dental fillings for almost 40 years, there remains a critical need for a significantly improved restorative material. Thiol-ene materials represent an ideal candidate for dental restorative materials in many ways. Not only is the polymerization not susceptible to water or oxygen inhibition, but it also proceeds just as rapidly and more efficiently than a methacrylate homopolymerization. The mechanism of this reaction is significantly different, resulting in a material that has reduced shrinkage and stress and esters have recently been removed from several thiol-ene monomers to reduce their degradation. There are a wide variety of available starting materials for a thiol-ene polymerization, and the reaction proceeds without interacting with other functional groups. Thus, monomers can be chosen to reduce water sorption or hydrolytic/enzymatic degradation and toxicity. Even with all these advantages, more work needs to be done to improve the practicality of using this system in a dental composite. Thiol and alkene monomers can spontaneously react without any intended initiation action ? reducing the shelf-life of monomer mixtures. It is also well known that some thiol molecules are accompanied by an unpleasant odor. For a variety of reasons secondary thiol monomers are less likely to spontaneously react and also have a reduced odor. Although, there is still little information regarding how the substitution of the thiol will affect the thiol-ene reaction. Therefore, this project proposes to make an improved dental restorative material by incorporating secondary and tertiary thiol monomers into a thiol-ene composite. In order to do so, thorough kinetic and polymer studies must be completed so the effect from the more highly substituted thiols can be assessed. New primary, secondary, and tertiary thiol monomers will be synthesized and a library of thiol and alkene monomers will be evaluated for the best incorporation into dental restorative composites. These composites will be evaluated for improved mechanical and clinical properties against a commonly used dental composite.